A power system may include distributed power sources (e.g., distributed generators, battery banks, and/or renewable resources such as solar panels or wind turbines to provide power supply to a grid (e.g., a microgrid having local loads and/or a utility grid). The power system may include a power converter, such as a power inverter, for converting power between a power source and a grid. Such power conversion may include AC/DC, DC/DC, AC/AC and DC/AC.
A microgrid system can include a variety of interconnected distributed energy resources (e.g., power generators and energy storage units) and loads. The microgrid system may be coupled to the main utility grid through switches such as circuit breakers, semiconductor switches (such as thyristors and IGBTs) and/or contactors. In the event that the microgrid system is connected to the main utility grid, the main utility grid may supply power to the local loads of the microgrid system. The main utility grid itself may power the local loads, or the main utility grid may be used in combination with the power sources of the microgrid to power the local loads.
A controller comprising hardware and software systems may be employed to control and manage the microgrid system. Furthermore, the controller may be able to control the on and off state of the switches so that the microgrid system can be connected to or disconnected from the main grid accordingly. The grid connected operation of the microgrid system is commonly referred to as “grid tied” mode, whereas the grid disconnected operation is commonly referred to as “islanded” or “stand alone” mode. A microgrid system in grid-tied mode should be capable of being disconnected from the main grid and transitioning to islanded mode in the case of a grid event in which abnormal operation conditions, such as a power outage, occur at the main utility grid.
When the microgrid includes a battery bank, a battery energy storage system may be used to provide power to, or to receive power from, the microgrid. The battery energy storage system can be used as an energy storage unit in a smart grid system. Renewable energy sources such as photovoltaic/solar panels and wind turbines are intermittent sources subject to unpredictable and inconvenient weather patterns. The generation source rarely matches the load needs; and therefore, it is desirable to provide energy storage units. The use of energy storage units, which can both store and supply power, allows the microgrid system to provide reliable and stable power to local loads.
The energy storage units can also store excess energy from the renewable sources (and potentially the grid). For example, renewable energy generation may exceed load demand of the microgrid. Without energy storage capability, the extra generation is lost. If energy storage units are employed in the microgrid, the extra generation can be captured by storing it in the batteries. The energy storage units can then supply this power to local loads and even the main utility grid where appropriate.
Unfortunately, existing implementation do not enable multiple units to be seamlessly transitioned between grid-tied and stand-alone mode.